Turbines, such as those used in gas turbine engines, generally include a casing or shroud that houses a rotor configured to rotate therein. The rotor includes a hub and a plurality of circumferentially-spaced rotor blades extending outward from the hub. The components of the rotor are positioned, configured, and sized such that a relatively small gap or clearance exists between the tips of the rotor blades and the interior surface of the casing. The size of the clearance is directly related to the efficiency at which the turbine operates. During operation, however, the size of the clearance may vary due to a number of factors, such as the expansion and/or contraction of the components of the turbine due to temperature fluctuations, movement of the components of the turbine, and/or degradation of the tips of the rotor blades and/or the interior surface of the casing.
Since the size of the clearance between the tips of the rotor blades and the interior surface of the casing directly affects the efficiency of the turbine, and since the size of the clearance may vary during operation of the turbine and thus vary the efficiency of the turbine, it is desirable to monitor the size of the clearance to ensure optimal turbine operation. There are a variety of known clearance detection systems configured to determine the size of the clearance.
One known clearance detection system is described in U.S. Pat. No. 4,049,349. This known clearance detection system includes a skewed arrangement of optical fibers connected to a remote light-source and a light detector. The skewed arrangement of the optical fibers generates two diverging light beams. This known clearance detection system measures the time-delay between a target reflecting light from the first and the second light beams. The time-of-flight between the two beams is calibrated to the clearance between the target and the sensor head.
Another known clearance detection system is described in “Time-Of-Flight Tip-Clearance Measurements,” authored by H. S. Dhawal, A. P. Kurkov, and D. C. Janetzke and published in the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Jun. 20-24, 1999. This known clearance detection system includes a time of flight probe that incorporates two separate optical probes into a sensor head. The two probes are tilted equally with respect to the axis of the sensor head. The signal generated by a target that traverses across the skewed light beams is timed to extract the time of flight between the two beams. The measured time of flight is calibrated to the clearance between a target and the sensor.
Another known clearance detection system is described in U.S. Pat. No. 8,009,939. This known clearance detection system includes a plurality of optical fibers that project light of different wavelengths at a target. This known clearance detection system exploits the relative amplitudes of the wavelengths of the reflected light to calculate the clearance between the target and the sensor face.
Another known clearance detection system is described in U.S. Pat. No. 7,400,418. This known clearance detection system relies on the detection of the power spectrum due to interference between a beam of light reflected from a target and a reference beam into which a deliberate delay was introduced using an acousto-optical module. The interference pattern produced by the two beams depends on the clearance between the moving target and the collimator and the delay introduced into the reference beam by the frequency modulated acousto-optical module. In this clearance detection system, the intensity of the reflected light has to be matched with the delayed reference beam through an attenuator.
Another known clearance detection system is described in U.S. Pat. No. 8,624,604. This known clearance detection system includes a main waveguide and a reference element that is provided at a position intermediate the proximal and distal ends, or at the distal end, of the waveguide. The transmitter/receiver is arranged to transmit an electromagnetic signal through the main waveguide and receive a reflection of the transmitted electromagnetic signal from the reference element, the casing surface, and from a target, enabling the relative positioning of the reference element, the casing surface, and the target to be simultaneously determined.
Another known clearance detection system is described in U.S. Pat. No. 7,333,912. This known clearance detection system includes a reference geometry disposed on a first object having an otherwise continuous surface geometry and a sensor disposed on a second object, wherein the sensor is configured to generate a first signal representative of a first sensed parameter from the first object and a second signal representative of a second sensed parameter from the reference geometry. This known clearance detection system also includes a processing unit configured to process the first and second signals to estimate a clearance between the first and second objects based upon a measurement difference between the first and second sensed parameters.
Another known clearance detection system is described in U.S. Pat. No. 8,164,761. This known clearance detection system uses a reference beam and a signal beam that have different focal lengths or that diverge/converge at different rates. The beams are fixed to the stationary member and proximate to each other. The beams are projected across a clearance between the stationary member and a target toward the target. The reference and signal beams are reflected by the target when the target intersects the reference and signal beam, and the reflected reference and signal pulses are obtained. One or more features of the reflected reference pulse and the reflected signal pulse, such as a rise time of the pulses, a fall time of the pulses, a width of the pulses, and a delay between the reflected reference pulse and the reflected signal pulse (among other factors) are obtained. The width of the clearance is obtained using at least one of these factors.
There is a continuing need for new and improved clearance detection systems and methods for determining the clearance between the blade tip and the casing.